It is well known that the term "color" as applied to electromagnetic radiation represents in part the relative energy distribution of the radiation within the visible spectrum. That is, light providing a stimulus to the human eye, and having a particular energy distribution, may be perceived as a substantially different color then light of another energy distribution. Concepts relating to the characteristics of color and light waves are the subject of numerous well known texts, such as Principles of Color Technology, Meyer, Jr. and Saltzman (Wiley 1966), and The Measurement of Appearance, Hunter and Harold (Wiley 2nd Ed 1987).
In recent years, the capability of maintaining the "quality" of color has been of significant importance in various industries, such as, for example, the fields of graphic arts, photography and color film processing. With respect to the graphic arts fields, it is necessary, for example, to maintain appropriate color quality throughout a production run of a color printing sheet.
For purposes of performing sample testing and other activities in furtherance of maintaining color quality, it is necessary to first determine an appropriate means for "measuring" and "describing" color. A substantial amount of research has been performed during the past 50 years with respect to appropriate methods and standards for color measurement and description.
For purposes of describing color, and from a purely physical point of view, the production of color requires three things: a source of light, an object to be illuminated, and a means for perceiving the color of the object. The means for perceiving the color can be the human eye and brain or, alternatively, a photosensitive detector and associated auxiliary equipment utilized for detecting light.
The maintenance of quality standards in photography requires precise control of exposure, source intensity, development procedures and film characteristics, in addition to the control of environmental variables. Similarly, the maintenance of quality standards in graphic arts also involves consideration of some of the same parameters and variables. In general, it is desirable to provide a means for measuring color so as to assess the manner in which an image will appear to a human observer, or the manner in which an image will perform in a photographic or other type of reproduction printing operation.
One parameter widely used in the field of color technology for obtaining a quantitative measurement is typically characterized as optical "density." Described simplistically, when light is directed onto an object or object sample to be measured for color, the object may absorb a portion of the light energy, while correspondingly passing through or reflecting (if the object is opaque) other portions of the light. The color characteristics of the object sample will depend in part on the spectral characteristics of the object. That is, the effect of an object on light can be described by its spectral transmittance or reflectance curves (for transparent or opaque materials, respectively). These spectral characteristic curves indicate the fraction of the source light at each wave length transmitted by or reflected from the materials. Such curves are a means for describing the effect of an object on light in a manner similar to the use of a spectral energy distribution curve for describing the characteristics of a source of light.
For purposes of determining these spectral characteristics, a detector can be appropriately positioned to respond to the light transmitted through or reflected by the object sample. Such a detector can, for example, be in the form of a photovoltaic device. Such a device can produce a current output proportional to input light intensity over several orders of magnitude.
In accordance with conventional optical physics, it is known that the proportion of light incident to an object sample and absorbed by such a sample is independent of the light intensity. Accordingly, a quantitative indication of the spectral characteristics of an object sample can be defined as the transmittance or reflectance of the sample. That is, the transmittance of a substantially transparent object can be defined as the ratio of power transmitted over light power incident to the sample. Correspondingly, for an opaque object sample, the reflectance can be defined as the ratio of power reflected from the object over the incident light power.
For collimated light, these ratios can be expressed in terms of intensities rather than power. Furthermore, because of the nature of transmittance/reflectance and the optical characteristics of the human eye, it is advantageous to express these ratios in logarithmic form. Accordingly, the optical density of an object sample is typically defined as the negative logarithm to base 10 of the transmittance or reflectance. In accordance with the foregoing, if an object sample absorbed 90% of the light incident upon it, and the object were opaque, the reflectance would ideally be 10%. The density of such a sample would then be characterized as unity. Correspondingly, if 99.9% of the light were absorbed, the reflectance would be 0.1% and the density would be 3. Similarly, the density of an "ideal" object reflecting 100% of the light incident upon it would be 0.
To provide a relative measurement of color, it is possible to utilize the principles of density determinations without requiring measurement or knowledge of the absolute values of total incident light intensity or reflectance. That is, for example, it is possible to obtain relative color measurements among a series of object samples by utilizing a particular geometric configuration of light, object sample and reflectance or transmittance detector for each measurement, and standardizing the measurements in some desired manner.
In brief summary, optical density is a measurement of the modulation of light or other radiant flux by an object sample, such as a given area of printed ink-on-paper. Density measurements provide a means to assess the manner in which an image will appear to a human observer, or the way an image will perform in a printing operation. Density measurements can be utilized to produce sensitometric curves to evaluate various printing and reproduction characteristics, as well as utilization to control various photographic operations, such as film processing.
For purposes of measuring optical densities, it is well known to employ a device typically characterized as a densitometer. For purposes of further description of the background of the invention, additional discussion will be limited to principles associated with "reflection" densitometers, which are employed for optical density measurements of opaque objects. However, it should be emphasized that the principles of the invention are not limited to reflection densitometers, and are equally applicable to transmittance densitometers employed for determining the spectral characteristics of various none-opaque materials.
Reflection densitometers are utilized in the graphic arts for performing a variety of functions. As an example, it is common to provide color printing sheets with color bar strips extending along an edge of the sheet. When such a printed sheet has been approved for production, the optical color density of the color bars can be determined with the densitometer. Thereafter, during production runs, the color bars on the edges of the corresponding printed sheets can be checked with the densitometer, so as to assure that appropriate color densities are being maintained.
In addition, reflection densitometers can be employed in the area of photography. For example, such a densitometer can be utilized to determine the optical density of the brightest or "highlight" areas, and the darkest or "shadow" areas of a subject to be photographed. Such values can be utilized in adjusting controls of the camera so as to assure appropriate exposure.
Still further, reflection densitometers can be conveniently employed in color film processing. It is common for color film manufacturers to provide test strips having color bars or patches. If the test strips have been appropriately processed, the bars will have known densitometer readings. Such strips can then be utilized to check operating parameters of a film processing system, before the system is utilized to process the exposed film.
Correspondingly, transmittance densitometers can also be employed with respect to film processing. For example, test strips of negatives having color bars can also be employed. Again, if the test strips have been appropriately processed, the bars will have known densitometer readings. These strips can be utilized to check operating parameters of a film processing system.
In addition to concepts associated with reflection measurements and transmittance measurements, it is also known to employ densitometers for providing printer balance functions.
To assist in describing the principles of the invention, presently known techniques of measuring optical density can be illustrated by the schematic representation of a known reflection densitometer configuration 100 as shown in FIG. 1. Referring to the numerical references therein, the prior art reflection densitometer 100 includes a light source unit 102 having a source light 104. With respect to optical density measurements in photography and other industrial fields, various standards have been developed for densitometer illuminating light sources. For example, densitometer standards have previously been described in terms of a tungsten lamp providing an influx from a lamp operating at a Planckian distribution of 3,000K. Other suggested standards have been developed by the American National Standards Institute ("ANSI") and the International Organization for Standardization ("ISO"). These light source densitometry standards are typically defined in terms of the spectral energy distribution of the illuminant.
The source light 104 is directed through a collimating lens 106 which acts to converge the electromagnetic radiation from the source light 104 into substantially parallel rays of light. The light rays transmitted through the lens 106 are further directed through an aperture 108. The dimensions of the aperture 108 will determine the size of the irradiated area of the object sample under test. Various standards have been defined for preferable sizes of the irradiated area. Ideally, the aperture 108 would be of a size such that the irradiance is uniform over the entire irradiated area. Current standards suggest that the size of the irradiated area should be such that irradiance measured at any point within the area is at least 90% of the maximum value.
The light rays transmitted through aperture 108 (illustrated as rays 110 in FIG. 1) are projected onto the irradiated area surface of the object sample 112 under test. The sample 112 may be any of numerous types of colored opaque materials. For example, in the printing industry, the sample 112 may be an ink-on-paper sample comprising a portion of a color bar at the edge of a color printing sheet. However, as will be apparent from the subsequent description herein, the principles of the current invention are not limited to measurement of printed ink-on-paper, photography or other specific fields.
As the light rays 110 are projected onto the object sample 112, electromagnetic radiation shown as light rays 114 will be reflected from the sample 112. For purposes of determining the relative proportions of light reflected from various object samples, it is necessary to obtain a quantitative measurement of this reflected light. However, it is undesirable (and substantially impossible) to measure all of the light reflected from the sample 112. Accordingly, standard detection configurations have been developed whereby reflected light is detected at a specific angle relative to the illumination light rays 110 projected normal to the plane of the object sample 112. More specifically, standards have been developed for detection of reflected light rays at an angle of 45.degree. to the normal direction of the light rays 110.
For purposes of actual detection of the reflected light rays 114, a rotatable spectral filter apparatus 116 is provided. The filter apparatus 116 can include a series of filters 118, 120 and 122 which are employed for purposes of discriminating red, green and blue spectral responses, respectively. That is, each of the filters will tend to absorb light energy at frequencies outside of the bandwidth representative of the particular color hue of the filter. For example, the red filter 118 will tend to absorb all light rays except for those within the spectral bandwidth corresponding to a red hue and centered about a wavelength of approximately 610 namometers (nms). By detecting reflected light rays only within a particular color hue bandwidth, and obtaining an optical density measurement with respect to the same, a "figure of merit" can be obtained with respect to the quality of the object sample coloring associated with that particular hue.
It is apparent from the foregoing that the actual quantitative measurement of color density or reflectance is dependent in substantial part on the spectral transmittance characteristics of the filters. Accordingly, various well known standards have been developed with respect to spectral characteristics of densitometer filters. For example, one standard for densitometer filters is known as the ANSI status T color response. The spectral response characteristics of filters meeting this standard are relatively wide band (in the range of 50 to 60 namometer bandwidth) for each of the red, blue and green color hues. Other spectral response characteristic standards include, for example, what is known as G-Response, which is somewhat similar to status T, but is somewhat more sensitive with respect to denser yellow hues. An E-Response represents a European response standard.
The spectral filter apparatus 116 shown in FIG. 1 includes not only the filters 118, 120 and 122, but is also shown as including a shaft 124 having one end connected to a "wheel" 126 on which the spectral filters are positioned and spaced apart. The other end of the shaft is connected to a manually rotatable knob 128. In the actual mechanical configuration of the densitometer 100, the knob 128 would be made accessible to the user for purposes of manual rotation of the wheel 126, so as to selectively position the individual filters as desired. In FIG. 1, the red filter 118 as shown as being appropriately positioned for detecting the reflected light rays 114.
The spectral filters 118, 120 and 122 can be any of several specific types of spectral response filters. For example, the filters 118, 120 and 122 can comprise a series of conventional Wratten gelatin filters and infrared glass. However, various other types of filter arrangements can also be employed.
As further shown in FIG. 1, the portion of the reflected light rays 114 which pass through the filters of the spectral filter apparatus 116 (shown as light rays 130) impinge on a receptor surface of a photovoltaic sensor cell 132. The sensor 132 is a conventional photoelectric element adapted to detect the light rays 130 emanating through the particular one of the filters 118, 120 and 122 then positioned to receive the reflected light rays 114. The sensor 132 is further adapted to generate an electrical current on line pair 134, with the magnitude of the output line current being proportional to the intensity of the light rays 130 sensed by the sensor 132. Photoelectric elements suitable for use as sensor 132 are well known in the art and various types of commercially available sensors can be employed.
The sensor current output on line pair 134 is applied as an input signal to a conventional amplifier 136. The amplifier 136 serves to convert the electrical current signal on line pair 134 to an output voltage signal on line 138. The amplifier 136 can include gain adjustment circuitry (representatively shown as an adjustable resistance in FIG. 1) 139 for purposes of varying the output voltage to input current gain. For example, a standard may be defined for the densitometer density reading for a particular spectral filter for zero density level. Accordingly, the amplifier circuit 136 can be adjusted by means of the gain adjustment circuitry 139 so that the densitometer reading is appropriate for the standard.
The output voltage signal from the amplifier 136 on line 138 can be applied as an input signal to a logarithmic voltage converter 140. The logarithmic voltage converter 140 is adapted to provide an output on line 142 which corresponds to the optical density measurement for the object sample 112 and the particular configuration of the spectral filter arrangement 116. This optical density measurement may be in the form of the negative logarithm (to the base 10) of the ratio of the voltage signal on line 138 to a standardized voltage magnitude. This standardized voltage magnitude can be set to a value which the user wishes to have correspond to a zero optical density measurement. That is, if the output voltage on line 138 is equal in magnitude to the standardized value, the logarithmic computation provided by the logarithmic converter 140 would generate a density measurement on line 142 of zero.
Preferably, the logarithmic converter 140 also has gain adjustment circuitry 144. This gain adjustment circuitry 144 can be utilized to set the density "slope" sensitivity of the converter 140. As is well known in the art of densitometer circuit design, logarithmic converters can vary in the response characteristics to input voltages. The gain adjustment provides a means for adjusting the response characteristics.
The voltage output from the logarithmic voltage converter 140 on line 142 can be applied to any of numerous types of conventional display apparatus 146. The display apparatus 146 is utilized to provide a visual display to the user of the density measurement represented by the logarithmic converter output voltage on line 142.
Although the foregoing prior art densitometer 100 has been described with the logarithmic conversion and gain adjustment functions represented by discrete components, it is apparent that such functions can clearly be performed by means of a digital computer or other computer apparatus.
As is well known in the art, densitometer apparatus must first be "calibrated" to provide a desired density response characteristic for a given set of spectral filters. In known systems, for example, and as briefly discussed in previous paragraphs, the "zero density" condition and the response "slope" for a particular densitometer and filter set can be provided as parameters manually input to the densitometer. For example, to provide what can be characterized as an "initial condition" of zero density for each individual spectral filter, an object sample comprising a "white" reference patch (representing substantial reflection) can be measured for each of the individual filters. The densitometer gain adjustments can then be manually adjusted so as to provide a standardized densitometer reading for the patch. Correspondingly, with the logarithmic density measurement assumed to be linear, the "slope" of the densitometer response can be set by means of viewing a "black" patch (representing substantial absorption), and setting the densitometer reading to a standardized "maximum" for the patch measurement for each of the filters.
Although the foregoing represents a means for calibrating zero density level measurements and density slope sensitivity, the known systems employing these calibration procedures still suffer from several substantial disadvantages. First, when standards are provided for adjusting the density level readings for a particular filter types, the standards assume an "ideal" filter. However, any physically realizable spectral filter arrangement will vary from the ideal. For example, in a conventional Wrattan filter configuration, such errors may be within the range of +-5 namometers. Such filter manufacturing errors can correspondingly result in errors as large as + or -0.08 density units in measurement of certain printed ink types. Such errors are critical, since desired industry inter-instrument agreement is within + or -0.02 density.
In addition, historical data regarding density measurements can be of primary importance, especially within the printing industry. That is, all printing being performed within a singular controlled environment should be capable of measurement by a number of densitometers in a manner so that the same results are achieved for identical measurements. However, if a series of conventional densitometers were utilized to measure the same color area, and were calibrated in accordance with the previously described procedures, the densitometers would not display identical measurement readings. Accordingly, if one densitometer had been used for an extensive period of time and had generated important historical printing data, such data would be substantially useless if the densitometer malfunctioned and a second densitometer instrument were subsequently utilized.
Problems associated with previously known calibration procedures result from several other considerations, in addition to the problems associated with manufacturing tolerances of spectral filter arrangements. For example, specification standards for various types of spectral filter arrangements call for certain types of light and color temperature, in addition to other illuminant parameters. However, manufacturing errors exist with respect to all physically realized illuminants. Furthermore, as a densitometer is used over a period of time, filament lamps will tend to drift. Still further, manufacturing errors will tend to exist with respect to photovoltaic detectors and other densitometer components. All of these factors result in problems associated with calibration based on standard spectral responses and the use of multiple densitometers for measuring color within a singular environment.
A substantial advance in the development of densitometers and calibration techniques has been provided in a densitometer arrangement disclosed in the commonly assigned U.S. patent application Ser. No. 105,424 filed Oct. 5, 1987. A densitometer arrangement as disclosed in the commonly assigned application is shown in FIG. 2. Densitometer apparatus of the type shown in FIG. 2 are characterized as reflection densitometers and utilized to provide color density measurements of opaque materials as previously described.
Several of the elements of the densitometer apparatus 200 were previously described with respect to the conventional densitometer configuration 100, and will only briefly be described herein. Referring specifically to FIG. 2, and the numerical references therein, the densitometer apparatus 200 includes a light source unit 202 having a source light 204. Various standards have been developed for densitometer light source illuminants for optical density measurements in photography, printing and other industrial fields. For example, densitometer standards have previously been described in terms of a tungsten lamp providing an influx from a lamp operating at a Planckian distribution of 3000K. Other suggested standards have been developed by the American National Standards Institute (ANSI) and the International Organization for Standardization ("ISO"). These source light densitometry standards are typically defined in terms of the spectral energy distribution of the illuminant. The source light 204 preferably conforms to an appropriate standard and can, for example, comprise a filament bulb meeting a standard conventionally known in the industry as 2856K ANSI. Power for the source light 204 and other elements of the densitometer apparatus 200 can be provided by means of conventional rechargeable batteries or, alternatively, interconnection to AC utility power.
The source light 204 projects light through a collimating lens 206 which serves to focus the electromagnetic radiation from the source light 204 into a narrow collimated beam of light rays. Various types of conventional and well-known collimating lenses can be employed. The light rays transmitted through the collimating lens 206 project through an aperture 208. The dimensions of the aperture 208 will determine the size of the irradiated area of the object sample under test. Various standards have been defined for preferable sizes of the irradiated area. Ideally, the aperture 208 is of a size such that the irradiance is uniform over the entire irradiated area. However, in any physically realizable densitometer arrangement, such uniform irradiance cannot be achieved. Current standards suggest that the size of the irradiated area should be such that irradiance measured at any point within the area is at least 90 percent of the maximum value. In addition, however, aperture size is typically limited to the size of color bar areas to be measured, and is also sized so as to reduce stray light.
The light rays emerging from the aperture 208 (illustrated as rays 210 in FIG. 2) are projected onto the irradiated area surface of an object sample 212 under test. The sample 212 may be any of numerous types of colored opaque materials. For example, in the printing industry, the sample 212 may be an ink-on-paper sample comprising a portion of a color bar at the edge of a color printing sheet. However, as will be apparent from the subsequent description herein, the principles of the invention are not limited to particular fields.
As the light rays 210 are projected onto the object sample 212, electromagnetic radiation shown as light rays 214 will be reflected from the sample 212. Standard detection configurations have been developed, whereby reflected light is detected at a specific angle relative to the illumination light rays 210 projected normal to the plane of the object sample 212. More specifically, standards have been developed for detection of reflected light rays at an angle of 45.degree. to the normal direction of the light rays 110. This angle of 45.degree. has become a standard for reflectance measurement and is considered desirable in that this configuration will tend to maximize the density range of the measurements. In addition, however, the 45.degree. differential also represents somewhat of a relatively normal viewing configuration of a human observer (i.e. illumination at a 45.degree. angle from the viewer's line of sight).
For purposes of providing light detection, a spectral filter apparatus 216 is provided. The filter apparatus 216 can include a series of filters 218, 220 and 222. The filters 218, 220 and 222 are employed for purposes of discriminating the cyan, magenta and yellow spectral responses, respectively. That is, each of the filters will tend to absorb light energy at frequencies outside of the bandwidth representative of the particular color hue of the filter. For example, the cyan filter 218 will tend to absorb all light rays, except for those within the spectral bandwidth corresponding to a red hue. By detecting reflected light rays only within a particular color hue bandwidth, and obtaining an optical density measurement with respect to the same, a "figure of merit" can be obtained with respect to the quality of the object sample coloring associated with that particular color hue.
It is apparent from the foregoing that the actual quantitative measurement of color density of reflectance is dependent in substantial part on the spectral transmittance characteristics of the filters. Accordingly, various well-known standards have been developed with respect to spectral characteristics of densitometers filters. These standards were previously described with respect to the prior art densitometer apparatus 100 illustrated in FIG. 1.
Although the filters 218, 220 and 22 are illustrated in the embodiment shown in FIG. 2 as the cyan, magenta and yellow color shades, other color shades can clearly be employed. These particular shades are considered somewhat preferable in view of their relative permanence and because they comprise the preferred shades for use in reflection densitometer calibration. However, it is apparent that different shades of red, blue and yellow, as well as entirely different colors, can be utilized with the densitometer apparatus 200.
The spectral filters 218, 220 and 222 may not only comprise various shades of color, but can also be of any of several specific types of spectral response filters. For example, the filters can comprise a series of conventional Wratten gelatin filters and infrared glass. However, various other types of filter arrangements can also be employed.
The spectral filters 218, 220, 222 are preferably positioned at a 45.degree. angle relative to the normal direction from the plane of the object sample 212 under test. However, unlike the densitometer configuration 100 previously described, each of the filters 218, 220 and 222 are maintained stationary and are utilized to simultaneously receive light rays reflected from the object sample 212 under test. Accordingly, it is unnecessary for the user to manually rotate or otherwise sequentially move spectral filters into receptive positions. Various types of densitometer structural configurations can be utilized to appropriately position each of the filters at the preferable 45.degree. angular position.
As further shown in FIG. 2, the portion of the reflected light rays 214 which pass through the filters 218, 220 and 222 (shown as light rays 224, 226 and 228, respectively) impinge on receptor surfaces of photovoltaic sensor cells. The sensor cells are illustrated in FIG. 2 as sensors 232, 234 and 236 associated with the spectral filters 224, 226 and 228, respectively. The sensors 232, 234 and 236 can comprise conventional photoelectric elements adapted to detect the light rays emanating through the corresponding spectral filters. The sensors are further adapted to generate electrical currents having magnitudes proportional to the intensities of the sensed light rays. As illustrated in FIG. 2, the electrical current generated by the cyan sensor 232 in response to the detection of light rays projecting through the filter 218 is generated on line pair 238. Correspondingly, the electrical current generated by the magenta sensor 234 is applied to the line pair 240, while the electrical current generated by the yellow sensor 236 is applied as output current on line pair 242. Photoelectric elements suitable for use as sensors 236, 238 and 240 are well known in the art, and various types of commercially available sensors can be employed.
The magnitude of the electrical current on each of the respective line pairs will be proportional to the intensity of the reflected light rays which are transmitted through the corresponding spectral filter. These light rays will have a spectral distribution corresponding in part to the product of the spectral reflectance curve of the object sample 212, and the spectral response curve of the corresponding filter. Accordingly, for a particular color shade represented by the spectral response curve of the filter, the magnitude of the electrical current represents a quantitative measurement of the proportion of reflectance of the object sample 212 within the frequency spectrum of the color shade.
As further shown in FIG. 2, the sensor current output on each of the line pairs 238, 240 and 242 is applied as an input signal to one of three conventional amplifiers 244, 246 and 248. The amplifier 244 is responsive to the current output of cyan sensor 232 on line pair 238, while amplifier 246 is responsive to the sensor current output from magenta sensor 234 on line pair 240. Correspondingly, the amplifier 248 is responsive to the sensor current output from yellow sensor 236 on line pair 242. Each of the amplifiers 244, 246 and 248 provide a means for converting low level output current from the respective sensors on the corresponding line pairs to voltage level signals on conductors 250, 252 and 254, respectively. The voltage levels of the signals on the respective conductors are of a magnitude suitable for subsequent analog-to-digital (A/D) conversion functions. Such amplifiers are well known in the circuit design art and are commercially available with an appropriate volts per ampere conversion ratio, bandwidth and output voltage range. The magnitudes of the output voltages on lines 250, 252 and 254 again represent the intensity of reflected light rays transmitted through the corresponding spectral filters.
Each of the voltage signal outputs from the amplifiers is applied as an input signal to a conventional multiplexer 256. The multiplexer 256 operates so as to time multiplex the output signals from each of the amplifiers 244, 246 and 248 onto the conductive path 258. Timing for operation of the multiplexer 256 can be provided by means of clock signals from master clock 260 on conductive path 262. During an actual density measurement of an object sample, the densitometer 200 will utilize a segment of the resultant multiplexed signal which sequentially represents a voltage output signal from each of the amplifiers 244, 246 and 248.
The resultant multiplexed signal generated on the conductive path 258 is applied as an input signal to a conventional A/D converter 264. The A/D converter 264 comprises a means for converting the analog multiplexed signal on conductor 258 to a digital signal for purposes of subsequent processing by central processing unit (CPU) 266. The A/D converter 264 is preferably controlled by means of clock pulses applied on conductor 268 from the master clock 260. The clock pulses operate as "start" pulses for performance of the A/D conversion. The A/D converter 264 can be any suitable analog-to-digital circuit well known in the art and can, for example, comprise sixteen binary information bits, thereby providing a resolution of 64K levels per input signal.
The digital output signal from the A/D converter 264 is applied as a parallel set of binary information bits on conductive paths 270 to the central processing unit (CPU) 266. The CPU 266 can provide several functions associated with operation of the densitometer apparatus 200. In the embodiment described herein, the CPU 266 can be utilized to perform these functions by means of digital processing and computer programs. In addition, the CPU 266 can be under control of clock pulses generated from the master clock 260 on path 272. However, it should be emphasized that a number of the functional operations of CPU 266 could also be provided by means of discreet hardware components.
In part, the CPU 266 can be utilized to process information contained in the digital signals from the conductive paths 270. Certain of this processed information can be generated as output signals on conductive path 276 and applied as input signals to a conventional display circuit 278. The display circuit 278 provides a means for visual display of information to the user, and can be in the form of any one of several well known and commercially available display units.
In addition to the CPU 266 receiving digital information signals from the conductive paths 270, information signals can also be manually input and applied to the CPU 266 by means of a manually accessible keyboard circuit 280. The user can supply "adjustments" to color responses by means of entering information through the keyboard circuit 280. Signals representative of the manual input from the keyboard circuit 280 are applied as digital information signals to the CPU 266 by means of conductive path 282.
The previously described concepts of densitometry can be of primary significance in the color photography and processing industry. For purposes of illustration and example, the color photograph processing procedure can be described as comprising a series of three process steps. First, the exposed roll or strip of color film is subjected to a process for producing a series of "negatives" from the exposed film roll or strip. This process is well known in the photography industry and can essentially be characterized as a chemical process for producing a series of negative images, in which the "brightness" values of the photograph subject are reproduced so that the lightest areas are shown as the darkest areas.
Secondly, the color photography development process comprises a step wherein the photographic negative is utilized with photographic paper in a manner such that the photographic paper is subjected to exposure from the negative. In this process, the film base and exposure times can be varied as appropriate to achieve the proper color balance on the exposed paper. Finally, the exposed film paper is subjected to a chemical process for generating the finished photographic prints.
Each of the aforedescribed processes is relatively conventional and well known in the photographic industry. However, each of these processes requires the "setting" of various control variables on the equipment utilized to perform the processes. For example, the processes associated with producing the negatives and processing the exposed paper comprise chemical processes whereby color chemistry variables may be adjusted so as to produce negatives and finished prints of appropriate colors. Correspondingly, the process step whereby the photographic print paper is exposed from the negatives will also have various variables associated with the process. For example, this particular process will involve the use of "white" light sources and spectral filters for exposing the negative onto the photographic paper in differing manners. Further, a variable associated with this particular process comprises the exposure times for the exposure of the negative onto the photographic paper. As an example, the negative may be exposed onto the paper through an unfiltered white light source for a certain predetermined period of time. However, if such an exposure is not producing an appropriate color balance, filters may be employed whereby only a particular color (i.e. energy from a portion of the color spectrum) of the white light source is exposed onto the photographic paper for some portion of the entirety of the exposure time. This type of operation is typically referred to as a "balancing" of the color.
With respect to the final step of the photograph development process, i.e. the processing of the exposed photographic paper to produce the final photographic prints, a number of variables are also associated with this type of process. For example, the chemistry of the film bath may be varied through the use of various chemical mixtures so as to again achieve correct print processing to maintain appropriate photograph colors.
Various methods and equipment have been developed for providing the photograph developers with a means for measuring the "quality" of the individual process steps associated with the entirety of the photograph development process. In particular, it is relatively well known to utilize densitometers to measure optical transmittance density of processed negatives and optical reflectance density of processed photographic paper to determine if the equipment is producing appropriate color balances. However, when measuring color densities to determine the quality of the film processing, it is desirable to compare such density measurements against "ideal" processed materials. Accordingly, the field of film processing readily lends itself to the comparison of color densities of materials processed by the operator's own equipment against reference standards.
Further, however, the photography industry does not have any ideal standards relating to each of the process steps associated with film development. Additionally, optimum color densities of processed materials may vary dependent upon the particular type of film or paper material being utilized by the operator. Accordingly, manufacturers of film processing equipment and materials will provide their own individual reference standards for purposes of optimizing the film development process.
More specifically, it is known in the field of color photograph film processing to utilize "strips" of negative and paper materials to periodically test the quality of the operator's own processing equipment. In addition, manufacturers also provide "reference" strips of materials which can be characterized as processed strips comprising "ideal" processing of the manufacturers materials.
To further illustrate the use of the reference strips and the control strips, a strip commonly identified as the Kodak C-41 strip is illustrated in FIG. 4. The C-41 strip is manufactured by Eastman Kodak Company. The strip illustrated in FIG. 4 is identified as strip 400 and comprises a film negative having various color hues associated with the negative. When the film development equipment operator is utilizing film negatives manufactured by Eastman Kodak, the operator will obtain a reference film strip and a series of control strips having a configuration shown in FIG. 4. The reference strip can be characterized as a negative which has been fully processed by the manufacturer. The negative is considered to comprise a series of color patches having the "ideal" color hues for the negative processing. Correspondingly, the control strips provided by the manufacturer will be a series of unprocessed strip negatives. The principal use and concept associated with these strips is to allow the operator to adjust the film negative processor so that the color densities of control strips processed by the negative processor will optimally "match" color densities of the reference strip.
To perform the operation of measuring the quality of the negative processing, a densitometer can first be used to measure the transmission densities of the reference strip. Again, these transmission densities represent ideal densities to be achieved by the equipment negative processor. Although it would be possible to utilize color density values somehow identified on the reference strip, such values may not comprise the same density values which will be measured by the operator's own densitometer. That is, the "absolute values" of the color densities are not particularly important. Instead, the quality of the film negative processing by the operator's equipment will be indicated by the comparison of the measured color densities of a processed control strip relative to the measured color densities of the reference strip. Because densitometers may vary in their measurement readings from one device to another, it is of primary importance that the color densities for the reference strip and the control strips be measured by the same device.
After measurement of the color densities associated with the reference strip, a control strip having a similar configuration to the strip 400 is processed by the operator, using the operator's own equipment. Following processing of the film negative, the processed control strip is now measured to determine the color densities associated therewith. The differences in the relative color density measurement values between the reference strip and the processed control strip will indicate to the operator whether any adjustments in the film negative processing operation are required. Indeed, many of the primary manufacturers will provide written "troubleshooting" manuals indicating the types of adjustments which may be necessary in view of certain types of differences between the density measurements associated with the processed control strip and the density measurements associated with the reference strip. As an example, the operator may find that the "green" density value for the processed control strips is continuously lower than the green density value for the reference strip. The written troubleshooting manuals may then provide suggestions as to the particular activities which may be undertaken by the operator with respect to adjustment of the negative processor equipment.
With respect to adjustments to the processing equipment associated with the exposure of the negative onto the photographic paper, manufacturers provide reference and control strips commonly referred to as "print balance" strips. Such a print balance control strip is illustrated in FIG. 5 as print balance strip 402. As shown in FIG. 5, the strip comprises three color patches identified as the "over", "normal" and "under" patches. These patches comprise color densities which may be expected with respect to photographic paper that has been overexposed, normal and underexposed, respectively. The print balance control strips are employed to maintain a printing balance during the exposure of a negative onto the photographic paper. Again, in a similar manner with respect to the processing step associated with processing the negatives, the manufacturer will provide a print balance reference strip, in addition to a series of unprocessed print balance strips. The operator would again measure the color densities of the patches of the reference strip representative of overexposure, normal processing and underexposure. These color density values would then be compared against the actual color density values of materials processed by the operator's own equipment. These measurements can assist the operator in adjusting exposure times and filtering so as to achieve a proper color balance in exposing the negative onto the photographic paper.
With respect to the third step of the overall development process, i.e. the processing of the exposed photographic paper to obtain the final photographic prints, the manufacturers provide further reference and control strips to adjust variables in the processing step. A control strip commonly identified as the Kodak EP-2 strip (manufactured by the Eastman Kodak Company) is illustrated as control strip 404 in FIG. 6. Again, the operator would be provided with a reference strip having the "ideal" color densities. That is, the reference strip would comprise a strip of photographic print having the ideal color densities for this processing step. The operator would measure these reflection color densities and compare the densities against control strips processed by the operator's own equipment. Manufacturers provide written troubleshooting manuals for this processing step in a manner similar to the materials provided for the production of the film negatives. That is, differences in the measured color densities will typically indicate certain problems associated with this step of the film processing. As an example, a relatively substantial distinction in the color densities of particular color patches between a processed control strip and the reference strip may indicate that the bath temperature for the processing of the final photographic print is not appropriate.
The common use of control strips as previously described herein with respect to photographic processing raises several issues. For example, it can be noted that the entirety of the process as described above involves the measurement of optical transmission densities (for the negatives) and optical reflection densities (for the film paper). In addition, it is apparent that the measurement of the color densities of the reference strips and the control strips can involve a substantial amount of manual manipulation. Accordingly, it would clearly be advantageous to employ a densitometer having the combined functions of reflection density measurement and transmission density measurement. In addition, it would also be advantageous to provide a means for "automating" the density measurement functions, dependent upon the particular types of standardized reference and control strips being employed. For purposes of the description of the illustrative embodiment of the invention as subsequently disclosed herein, references to "control strips" will refer to both reference strips and control strips.